This invention relates to a spark plug having a shield which aligns either a single or a multiple ground electrode within a combustion chamber to aid in having substantially identical operational conditions in each cylinder of an internal combustion engine.
Spark plugs, such as disclosed in U.S. Pat. No. 2,899,479, having a protruded tip which extends into a combustion chamber and which are cooled by gases that enter the combustion chamber which operate in a satisfactory manner under most driving conditions. The exact location of the ground electrode in the combustion chamber has an effect on the effectiveness of cooling effect of such gases. To provide for the optimum effect of such cooling, the relationship between the ground electrode and the position of the threads on the shell of the spark plug and in the head of the combustion chamber must be matched to assure each cylinder has the same capability of responding to the combustion of fuel in its corresponding combustion chamber. When spark plugs are placed in high performance engines such as in race cars, the plugs are sorted and matched to assure that the position of the ground electrode on the spark plug in each combustion chamber is identical. However, when vehicles are mass produced the time and effort involved in the sorting and matching does not occur.
In an effort to provide for better combustion and reduce the emission of noxious gases into the atmosphere as required by the Clean Air Act of 1986 in the United States, the valves in some engines have increased in size to allow for a more controlled and efficient exhaust of gases from a combustion chamber. Unfortunately, the increase in size of the valves had resulted in a decrease in the size of spark plugs. The decrease in size of the spark plugs results in a decrease in the inside diameter of the metal bore of the spark plug which effectively reduces the ability of a spark plug to resist carbon build up or deposits on the spark plug. Carbon deposits on a spark plug may reduce the ignition efficiency and result in an increase in the creation of environmental pollutants. In addition, carbon build up may cause shunt firing due to the close proximity of the inside of the bore.